The present invention relates generally to the ability to provide a uniform application of polymeric diphenylmethane diisocyanate (pMDI) onto gypsum boards, cellulose gypsum panels and other surfaces. More particularly, the present invention relates to the use of rotary cylinder brush technology to provide a uniform application of pMDI onto cellulose/gypsum based substrates.
Exterior wall cladding is used as a barrier to keep exterior air and moisture out of the wall cavity. If water and moisture penetrate the wall cladding surface damage will result to the cladding board itself. Prior art exterior wall cladding was made out of gypsum sheathing or water resistant gypsum board. It was found that the application of pMDI to gypsum board greatly increased the board's strength and water resistance; however, early attempts at applying pMDI to cellulose/gypsum based substrate membranes have met with little success. The prior art method of applying the pMDI was to use a spray apparatus which atomized the pMDI so it could be applied to gypsum board. The spray technique has several problems. First, the spraying of the pMDI results in a non-uniform application of the coating which prevents the achieving uniform water resistance across the gypsum board. Second, the atomization of pMDI creates a health concern by introducing small particles of pMDI into the air that can be inhaled by persons in the vicinity. The disclosed invention applies the pMDI with an apparatus that provides a uniform coating across the gypsum board which results in an increase in water resistance.